It's not that simple to list your companion on the reservation. This will require either a call to Marriott or to the hotel direct. Also, when you make the Vegas reservation you will have guaranteed it with your credit card. In my experience, when your companion checks in they will require a credit card from them for incidentals and possibly the entire room charge. When you arrive make sure that you have them remove your companions card from the reservation and add yours to make sure you receive the points and any stay credit.